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Full text of "Handbook Of Chemical Engineering - I"

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290                               CHEMICAL ENGINEERING
tinguished from the mode of waterproofing by rubber and gums which interfere with the ventilation of the fabric, and which is reputed to be an improvement on the chemical process of water repelling of treating the fabric with solution of sodium oleate followed by a bath of aluminum salt to form a metallic soap in the threads, consists essentially of an apparatus provided with graphite and aluminum electrodes between which and in close contact with which the fabric passes. The fabric before passing between the electrodes is saturated with a solution of sodium oleate while aluminum acetate flows down through grooves in the faces of the graphite electrodes. This saturates the fabric being treated and woolen pads which enclose the positive or aluminum pole. Electrolysis is set up and the resulting aluminum hydroxide is forced by cataphoresis through the woolen pad and through the fabric and tends to move towards the negative or graphite pole. In its passage through the fabric it reacts with the oleate and forms an insoluble basic aluminum oleate.
It is said that the chemical process does not firmly anchor the metal soap to the threads and that with use of the garment they become loosened from their anchorages. The ordinary aluminum oleate formed by the chemical process is soluble in the fluids used in dry cleaning, but the basic is not. Cataphoresis drives the aluminum hydrate particles into every part of the fabric. Tests on untreated cotton bags show leakage at the bottom in 1J^ seconds, those treated by the chemical process analagous to the electric treatment in 31 seconds while those treated by cataphoresis showed no leakage from the bottom at the end of three weeks. On putting samples of cotton in dye liquor the untreated cotton picked up very little color, the chemically treated cotton became a deep red while the electrically waterproofed showed an even deeper shade. The electric treatment is said to increase the strength and luster of the fabric. ("Electrolytic Water Proofing of Textile Fabrics/' by H. J. M. Creighton, Franklin Inst., Oct., 1921.)
Osmosis and Dialysis.—The phenomena of osmosis and the method of employing them belong rather to the field of pure chemistry than chemical engineering but the phenomena of electro-osmosis seems suggestive and will be touched upon briefly. It was Porret who discovered that if a strong current is passed into liquids as though they were to be electrolysed that there would be mechanical transport of the part of the liquid which becomes apparent if a porous diaphragm separates the electrodes by the liquid on one side of the diaphragm standing higher than the other. The phenomenon is most manifest with badly conducting liquids. The transfer of liquid particles is in the direction of the current and the liquid will stand highest on the cathode side of the diaphragm. Users of cells for the formation of sodium chloride and hydrate are familiar with electro-osmosis phenomena. If an anode side of a cell contains particles which would be negatively charged the liquid would migrate to the cathode and the solid particles to the anode.
Filters and Filtration.—In stationary or batch filters it is very seldom that the solids to be separated can be used as a medium and as is evident even in such cases there must be a support for the solids. The solids themselves do act as a medium to a certain extent even when prepared media are used. Prepared media range all the way through paper, cotton, and wool materials, biscuits of earthy materials, asbestos and slag wool, specially woven metal screens, to rock gravel and sand. Means of increasing the rapidity of filtration over the effect by gravity consist in applying suction or direct pressure by compressed air or steam or by mechanical appliances for exerting a direct pressure on the material being filtered. Surrounding the filter with hot water or a steam jacket will often